This invention relates to line sensors; and more particularly to a line integrated combination magnetic and strain line sensor.
There are many instances where one wishes to protect a given area against intrusion by a person or by an object such as a vehicle or the like. Many different types of alarm systems and detectors exist for protecting a given area. One such system is a line sensor. Line sensors are buried in the ground of the area to be protected and respond to any intrusion into the area. Both strain responsive and magnetic responsive line sensors are utilized.
The prior-art strain sensors commonly use a coaxial cable (strain cables) responsive to soil transmitted strain. The response between the center conductor and outer braid of the cable results in an analog signal, generated on the inner conductor, which can be processed electronically to actuate an alarm.
Prior art magnetic sensors commonly utilize a buried passive magnetic loop. Such loops are generally 300 meters long, coplanar with the earth's surface and are transposed at intervals. The transpositions are provided to nullify geomagnetic perturbations which can induce nuisance alarms. While such magnetic sensors have proved effective, they have inherent deficiencies which can limit their effectiveness. For example, if there is any unbalance in opposing loop areas, a net noise "Capture" area exists and this results in system vulnerability to nuisance alarming caused by lightning and other sources of geomagnetic noise. In order to minimize such false alarming, precise loop balance is required and such loop balancing can be accomplished only by trained personnel. Further, due to the required transpositions, these prior art loops are not readily buried in the ground since they require a multitude of cross trenches at five feet intervals.
This invention provides a combination magnetic and strain line sensor that is readily installed and operated by relatively unskilled personnel and is not as vulnerable to nuisance alarming caused by lightning or other geomagnetic noise as the prior transposed magnetic loop systems.